synch.h

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// synch.h 
//      Data structures for synchronizing threads.
//
//      Three kinds of synchronization are defined here: semaphores,
//      locks, and condition variables.
//
//      Note that all the synchronization objects take a "name" as
//      part of the initialization.  This is solely for debugging purposes.
//
// Copyright (c) 1992-1996 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// synch.h -- synchronization primitives.  

#ifndef SYNCH_H
#define SYNCH_H

#include "copyright.h"
#include "thread.h"
#include "list.h"
#include "main.h"

// The following class defines a "semaphore" whose value is a non-negative
// integer.  The semaphore has only two operations P() and V():
//
//      P() -- waits until value > 0, then decrement
//
//      V() -- increment, waking up a thread waiting in P() if necessary
// 
// Note that the interface does *not* allow a thread to read the value of 
// the semaphore directly -- even if you did read the value, the
// only thing you would know is what the value used to be.  You don't
// know what the value is now, because by the time you get the value
// into a register, a context switch might have occurred,
// and some other thread might have called P or V, so the true value might
// now be different.

class Semaphore {
  public:
    Semaphore(char* debugName, int initialValue);       // set initial value
    ~Semaphore();                                       // de-allocate semaphore
    char* getName() { return name;}                     // debugging assist
    
    void P();           // these are the only operations on a semaphore
    void V();           // they are both *atomic*
    void SelfTest();    // test routine for semaphore implementation
    
  private:
    char* name;        // useful for debugging
    int value;         // semaphore value, always >= 0
    List<Thread *> *queue;     
                        // threads waiting in P() for the value to be > 0
   };

// The following class defines a "lock".  A lock can be BUSY or FREE.
// There are only two operations allowed on a lock: 
//
//      Acquire -- wait until the lock is FREE, then set it to BUSY
//
//      Release -- set lock to be FREE, waking up a thread waiting
//              in Acquire if necessary
//
// In addition, by convention, only the thread that acquired the lock
// may release it.  As with semaphores, you can't read the lock value
// (because the value might change immediately after you read it).  

class Lock {
  public:
    Lock(char* debugName);      // initialize lock to be FREE
    ~Lock();                    // deallocate lock
    char* getName() { return name; }    // debugging assist

    void Acquire();             // these are the only operations on a lock
    void Release();             // they are both *atomic*

    bool IsHeldByCurrentThread() { 
                return lockHolder == kernel->currentThread; }
                                // return true if the current thread 
                                // holds this lock.
    
    // Note: SelfTest routine provided by SynchList
    
  private:
    char *name;                 // debugging assist
    Thread *lockHolder;         // thread currently holding lock
    Semaphore *semaphore;       // we use a semaphore to implement lock
};

// The following class defines a "condition variable".  A condition
// variable does not have a value, but threads may be queued, waiting
// on the variable.
//
// All operations on a condition variable must be made while
// the current thread has acquired a lock.  Indeed, all accesses
// to a given condition variable must be protected by the same lock.
// In other words, mutual exclusion must be enforced among threads calling
// the condition variable operations.
//
// These are only operations on a condition variable: 
//
//      Wait() -- release the lock, relinquish the CPU until signaled, 
//              then re-acquire the lock
//
//      Signal() -- wake up a thread, if there are any waiting on 
//              the condition
//
//      Broadcast() -- wake up all threads waiting on the condition
//
// In Nachos, condition variables are assumed to obey *Mesa*-style
// semantics.  When a Signal or Broadcast wakes up another thread,
// it simply puts the thread on the ready list, and it is the responsibility
// of the woken thread to re-acquire the lock (this re-acquire is
// taken care of within Wait()).  By contrast, some define condition
// variables according to *Hoare*-style semantics -- where the signalling
// thread gives up control over the lock and the CPU to the woken thread,
// which runs immediately and gives back control over the lock to the 
// signaller when the woken thread leaves the critical section.
//
// The consequence of using Mesa-style semantics is that some other thread
// can acquire the lock, and change data structures, before the woken
// thread gets a chance to run.  The advantage to Mesa-style semantics
// is that it is a lot easier to implement than Hoare-style.

class Condition {
  public:
    Condition(char* debugName, Lock * conditionLock);   // initialize condition to 
                                        // "no one waiting"
    ~Condition();                       // deallocate the condition
    char* getName() { return (name); }
    
    void Wait();        // these are the 3 operations on 
                                        // condition variables; releasing the 
                                        // lock and going to sleep are 
                                        // *atomic* in Wait()
    void Signal();   // conditionLock must be held by
    void Broadcast();// the currentThread for all of 
                                        // these operations
    // SelfTest routine provided by SyncLists

  private:
    char* name;
    Lock * condLock;
    List<Semaphore *> *waitQueue;       // list of waiting threads
};
#endif // SYNCH_H

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